1. Field of the Invention
The present invention relates generally to temperature regulation in a cryostat, with an exemplary purpose of the use of such a cryostat as an apparatus and a method for regulating temperature in a cryogenic measurement chamber, while cooling a superconducting magnet, using a cryogenic cooler as a source of refrigeration.
2. Discussion of the Prior Art
Systems have been available to employ cryostats for temperature regulation in the cryogenic temperature region. One use of such cryostats is to test the physical properties of specimens. The need for testing physical properties of specimens of various types for different properties has increased substantially over the last several years. Systems exist for characterizing the physical properties of various materials under variable measurement conditions by programming an arbitrary sequence of temperature and magnetic field sweeps and steps at which to characterize various physical properties of the sample specimen.
Such systems typically include a cryogenic chamber that has a number of heat shields, a coolant such as helium, a source of refrigeration (cryogenic cooler), a superconducting magnet, a sample chamber, and an apparatus for controlling temperatures, all of which may be referred to as a cryostat. Temperature regulation in a cryogenic test chamber requires a sophisticated balance between supply and loss of thermal energy, and various methods have been devised to accomplish such tasks at low (cryogenic) temperatures. A measure of the efficiency of a specific control scheme is the width of the temperature range over which control can be effectively and efficiently maintained, and the duration and stability achieved at any temperature in this range. An additional measure of the overall system performance is the amount of coolant usage, with lower usage rates being preferred.
One example of such a measurement system utilizes variable temperature field control apparatus, designed to perform a variety of automated measurements. In order to carry out the experiments the system is required to rapidly vary the magnetic fields generally between ±16 Tesla, while maintaining the magnet generally at a constant temperature of about 4.2 K. At the same time, a chamber containing a sample specimen and associated experimental apparatus is typically controlled at an arbitrary sequence of temperatures ranging from about 400 K to below about 2 K. This functionality necessitates a system design that is capable of delivering various amounts of cooling power at different temperatures to different components of the system. In addition, a typical test schedule requires achieving sample temperatures that are below the coldest stage of a typical cryogenic cooler (4.2 K under most practical conditions) and, therefore, employs the process of evaporation of a continuous stream of liquid helium.
Typically, a Gifford McMahon (GM) or a GM-type pulse-tube cryogenic cooler (PTC) is used for this purpose. PT cryogenic coolers provide different amounts of cooling power when operating at different temperature stages. The higher temperature stages provide substantially higher cooling power than the lower temperature stages. An example of such cryogenic cooler is the PT410, sold by Cryomech Inc, of Syracuse, N.Y., which may provide about 40 W of cooling power at the 50 K temperature stage, but only about one watt of cooling power at the 4.2 K stage.
Some presently available designs address the need for providing variable cooling power to the superconducting magnet and the sample chamber by employing a multistage PTC (three or more stages) together with a combination of various methods for coupling the cryogenic cooler to the rest of the cryostat assembly. Flexible braided metal links between the PTC and other elements of the cryostat, such as fixed heat exchanger units, are often used to physically couple PTC cooling elements to the rest of the cryostat. The use of flexible physical links or fixed heat exchangers limits modularity and uses of the measurement system, as the physical links place an upper limit on the heat exchange between the PTC and the other cryostat elements and additional thermal couplings may be necessary if increased heat exchange rate is required. Overall, physical coupling between the cryogenic cooler and the rest of the cryostat substantially complicates maintenance and increases the overall system complexity and cost.
Typically pulse-tube cryogenic cooler units generate vibrations at around 1 Hz frequency under normal operating conditions. Therefore, a system employing physical links transfers superfluous vibration energy from the PTC into the sample area which can be detrimental in applications that are particularly sensitive to small motions, such as optical interferometry, where special care needs to be taken to prevent vibration energy of the PTC from contaminating the sample signal. Efforts have been made to decouple sample signals from the vibration motions of the PTC.
Some presently available cryogenic measurement systems utilize separate re-condenser modules in order to convert gaseous coolant into liquid form that is typically required for cryostat operation at lowest temperatures. This approach increases system complexity and costs while limiting flexibility of use as the re-condenser unit needs to be in physical contact with the PTC. It is recognized in the art that multiple (or multiple stage) cryogenic cooler units are typically required to obtain very low temperatures of about 4.2 K or below.
The challenging task of connecting and disconnecting different cryogenic cooler stages at different temperatures is solved in one prior art example by employing a cryogenic cooler apparatus with at least three stages in combination with multiple heat exchangers and conduits to deliver cooling power from different stages to the desired areas in the cryostat.
Other prior art teaches that at least in theory mechanical valves may be used to open and close the coupling tubes during operation of the multistage cryogenic cooler to regulate cooling power distribution. However, the difficulty of constructing reliable low temperature valves has limited the usefulness of this approach. An alternate method for regulating temperature in the cryogenic chamber uses a dual capillary inlet chamber and multistage cooler/heater apparatus. Although such design allows for smooth temperature regulations in the sample chamber over the desired range it increases cost and complexity of the measurement apparatus and does not address the need of delivering additional cooling power to the superconducting magnet when the latter is operated in sweeping mode.